This invention relates to a vehicle suspension comprising leaf springs, in particular a suspension for a commercial vehicle, and a leaf spring for use in such a suspension.
In general, most commercial vehicles with a frame comprising longitudinal beams are equipped with steerable, single, tandem or multi-axle assemblies.
Conventionally, the suspension systems provided for supporting and damping the relative movement between each axle and the vehicle frame have included single-stage multi-leaf springs, pneumatic spring systems or a combination thereof. Many vehicles are equipped with single-stage multi-leaf springs which are designed to mechanically dampen the movement between the frame and die axles during operation of the vehicle.
It is common to support such a vehicle with a leaf spring which is attached at each end to the vehicle chassis and to an axle near the centre of the spring. In such an arrangement, when the suspension is loaded, the maximum bending moment in the spring occurs at the point where the axle is attached to the spring and decreases in either direction from that point along the spring. In order to make the most efficient use of the spring material and to save weight, one option is to use a single leaf spring. The spring can be made tapered in either direction toward its ends from a point of maximum thickness where the axle is attached. In order to achieve acceptable deflection characteristics in a tapered spring, it is necessary that it be manufactured to a relatively high degree of accuracy. The cost of manufacturing a tapered steel leaf spring is relatively high because it requires the use of special tapered rolling machines. As a result, although tapered single leaf springs are generally available today for use with larger vehicles, such as heavy trucks, they are significantly more expensive than conventional flat springs.
Another important factor in the design of leaf spring suspensions is the desired spring rate. The spring rate, which is defined as the rate of increase of force necessary to deflect the spring with deflection, is a function of the cross-sectional area moment of inertia, the length of the spring, and the elastic modulus of the spring material. In general, a single leaf spring must be designed so that it is strong enough to withstand the loads imposed upon it in operation and yet have a spring rate which is low enough to provide acceptable ride qualities. Further, the desired spring rate must be achieved within the particular geometric constraints placed on the suspension, such as the maximum allowable length and deflection of the springs.
To achieve a compromise among these various design and economic factors, designers frequently use “built-up” steel spring assemblies which consist of a number of separate spring leaves diminishing in length from the top of the assembly to the bottom to achieve an overall tapered shape. Normally, the leaves are clamped together at their centres where the axle is mounted but are free to slip longitudinally relative to each other when the spring is deflected. The leaves are usually of constant thickness to reduce manufacturing costs, but use of tapered leaves obtains greater efficiency. As previously mentioned, however, the cost of manufacturing such tapered leaves is relatively high.
One of the principal deficiencies of a built-up steel spring assembly is its weight. Due to the dramatic increase in fuel costs in recent years and the consequent necessity to reduce vehicle weight, designers are examining all major vehicle components, including suspensions, to see if ways can be found to reduce their weight without adversely affecting their cost or performance. In particular, it has been suggested that much lighter and more efficient springs could be made from various state-of-the-art plastic or composite materials rather than steel. Some of these materials are particularly attractive for use in constructing springs with non-uniform cross-sections because of the ease with which they can be moulded.
In spite of these efforts, plastic and composite single leaf springs have not been used commercially for a variety of reasons. It is generally accepted that springs made entirely of plastic would be impractical because of excessive bulk and insufficient resistance to wear and impact. One known plastic spring, sold under the trademark GRAETEK by a division of Exxon Corp., is made from graphite skins with a glass fibre-reinforced epoxy core. This spring has proven impractical due to high cost and its extreme unidirectional stress-carrying capability. That is, the spring is strong, enough in the vertical direction but too weak in the transverse or torsional direction to be usable in common suspensions. Since vehicle springs must absorb cornering loads and high impact loads from rocks and other debris, graphite is unsatisfactory as a component of a viable leaf spring. Composites of metal and plastic have been suggested to alleviate some of these graphite problems. The cost of manufacturing these composite springs has this far been too great, however, to justify substituting them for all steel spring assemblies.
An additional drawback with single leaf springs is that the installation must be secured by a safety system or device at each end, in case of spring, failure. For a steered front axle, the front end is provided with a safety eye to block a front axle dislocation and the rear end is provided with a steel belt or blocker. Such a safety system increases both cost and weight and give a minor axle dislocation at spring breakage that could lead to problems to maintain steering control after a leaf spring breakage. After a spring breakage the vehicle can not be driven and must be towed to a service facility.
In the subsequent text, the vehicle referred to is a commercial type vehicle comprising a frame built up of a pair of substantially parallel beams, for instance beams with an I- or C-shaped cross-section. The suspension according to the invention is preferably, but not necessarily, intended for front wheel suspensions comprising steerable wheels. It should be noted that all distances referred to are taken when the vehicle is stationary and the suspension is in its unloaded state, unless otherwise specified.
The invention relates, according to an aspect thereof, to a leaf spring assembly for a vehicle suspension arranged to be mounted in the longitudinal direction of the vehicle, on both sides thereof. Each leaf spring assembly is attached on opposite sides of a chassis or frame and is arranged to support one end of a transverse rigid axle. The axle is preferably, but not necessarily, a steered vehicle axle.
The leaf spring assembly has a first end, which is arranged for pivotal connection to a first bracket on the vehicle and a second end, which is arranged for connection to a spring shackle on the vehicle. In a conventional vehicle suspension, the first bracket is a front spring hanger or bracket and the second bracket is a rear spring hanger or bracket. As indicated above, the leaf spring assembly is arranged to be connected to an axle extending transversely of said leaf spring assembly at a position intermediate the first and second ends of said leaf spring assembly. The end of each spring comprises a spring eye, also termed an eye wrap.
According to the invention, the leaf spring assembly comprises two individual leaf springs arranged side-by-side with a predetermined, spacing and extending between said first and second ends. The individual leaf springs are preferably identical parabolic leaf springs.
The leaf spring assembly is arranged to replace a conventional single leaf spring, where the total width of the leaf spring assembly is equal to the width of the conventional single leaf spring. In this way the leaf spring assembly can be mounted to existing first and second brackets, with a minimum of modification. In this context, a single leaf spring, is defined as one solid leaf spring extending between said brackets, while a leaf spring assembly is defined as a pair of spaced and substantially parallel, individual leaf springs. The leaf spring assembly according to the invention preferably comprises parabolic springs, where the width and height of each parabolic spring can vary along its extension.
The individual leaf springs have a predetermined spacing and a width selected to provide a predetermined total width of the leaf spring assembly. The spacing can be constant or vary along the length of the leaf spring assembly. For instance, the spacing can vary depending on the selected edge radius and cross-section of the individual leaf springs. Preferably, the individual leaf springs has a width, thickness and spacing at any position along their extension between the spring eyes, selected so that the combined second moment of inertia of the leaf spring assembly is at least equal to that of a single leaf spring with the same total width at the same position.
The second moment of area Ixt; Iyt in the x- and y-planes through a central longitudinal axis for a single, solid leaf spring with a rectangular cross section is defined as:
                              I                      x            ⁢                                                  ⁢            1                          =                                            b              1                        ⁢                          h              1              3                                12                                    (        1        )                                          I                      y            ⁢                                                  ⁢            1                          =                                            h              1                        ⁢                          b              1              3                                12                                    (        2        )            where bi is the width and is height, or thickness, of the solid leaf spring.
For a spring assembly according to the invention, second moment of area I&, ly2 is defined as:
                              I                      x            ⁢                                                  ⁢            2                          =                  2          ×                                                    b                2                            ⁢                              h                2                3                                      12                                              (        3        )                                          I                      y            ⁢                                                  ⁢            2                          =                  2          ×                      (                                          (                                                                            h                      2                                        ⁢                                          b                      2                      3                                                        12                                )                            +                              (                                                      b                    2                                    ⁢                                      h                    2                                    ⁢                                      e                    2                    2                                                  )                                      )                                              (        4        )                                          where          ⁢                                          ⁢                      e            2                          =                              (                                          b                1                            2                        )                    -                      (                                          b                2                            2                        )                                              (        5        )            where b2 is the width and h2 is height, or thickness, of each of the two individual leaf springs.
FIG. 6 shows schematic cross-sections in a vertical plane, at right angles to the main direction of a prior an spring and a spring assembly according to the invention. The above measurements are indicated in the respective cross-section, shown as comprising rectangular shapes for simplicity.
For example, using the above equations (1)-(5), a standard single leaf spring can have a width bi of 100 mm and a thickness \\ of 10 mm. If the standard spring is to be replaced by a spring assembly having, the same total width bi of 100 mm, individual widths b2 of 29 mm and a spacing b3 of 42 mm, then the thickness h2 must be at least 12 mm. In this case the increase in thickness is determined by 1×2, as the side-by-side spring arrangement only has a limited effect on ly2 in the vertical plane. In this example the vertical plane is arranged at the thickest point of a spring assembly comprising two parabolic springs, which point is located adjacent the attachment point of the axle in this case.
Using the above values it can be calculated that Iχ2 is marginally larger than lxi while the value for ly2 is 111% of the value for lyi. Also, the total cross-sectional area A2 of the two springs in the spring assembly is only 70% of
the cross-sectional area Ai of the standard spring. Consequently, by using a spring assembly according to the invention the weight saving on each side of the vehicle for the above example is approximately 30%. From this it can be seen that a spring assembly according to the invention can replace a standard solid leaf spring, requiring an increase in thickness limited to a few millimeters. The standard brackets and spring shackles can also be retained. In order to achieve a substantial weight saving without a significant increase in spring thickness, the individual widths b2 of the inventive leaf springs is preferably, but not necessarily, selected in the range of 25% to 45% of the total width bi, more preferably within the range of 30% to 40% of the total width bi. The minimum size of the gap between the individual leaf springs is limited by the size or diameter of a locator pin for maintaining the predetermined spacing between the springs. The locator pin itself can be used as a spacer, or be provided with a cylindrical spacer placed onto the locator pin.
Each individual leaf spring comprises a spring eye, or eye wrap, at the first end the leaf spring assembly and is mounted on a common bushing. The bushing is substantially cylindrical and comprises a central first spacer with a width equal to the predetermined spacing. The first spacer can be in the form of a radial flange which has opposed annular contact surfaces in contact with a side surface of a spring eye on the respective leaf spring.
The individual leaf springs are arranged side-by-side with a predetermined spacing at the intermediate position, separated by second spacer with a width equal to the predetermined spacing. The second spacer is preferably part of an axle assembly used for attaching the transverse axle to the leaf spring assembly.
The invention also relates to a vehicle suspension comprising a pair of leaf spring assemblies arranged to extend longitudinally on opposed sides of a vehicle frame. Each leaf spring assembly has a first end pivotally connected to the vehicle with a first bracket attached rigidly to the frame at a first position. A second end of the leaf spring assembly is connected to the vehicle frame with a spring shackle pivotably connected to a second bracket attached rigidly to the frame at a second position. The spring shackle is provided to compensate for length changes of the leaf spring under load conditions. The suspension further comprises an axle extending transversely of the vehicle frame which axle is mounted to each leaf spring at a position intermediate its first and second ends. A damper means, such as an air spring or suspension strut, is mounted between the axle and the vehicle frame. The leaf spring assembly according to the invention comprises two individual leaf springs arranged side-by-side with a predetermined spacing and extending between said first and second ends.
As stated above, the leaf spring assembly is arranged to replace a conventional single leaf spring, where the total width of the leaf spring assembly is equal to the width of the conventional single leaf spring. The leaf springs of the assembly have a predetermined spacing and width selected to provide a predetermined total width of the leaf spring, assembly. Also, the individual leaf springs have a width and thickness at any position along their extension selected so that the combined second moment of inertia lx; ly of the leaf spring assembly is at least equal to that of a single leaf spring with the same total width at the same position. An example describing the effect of the spaced parallel leaf springs on the mechanical properties of the spring assembly is given above. From the above it can be shown that a spring assembly according to the invention can replace a standard solid leaf spring, requiring an increase in thickness limited to a few millimeters. The standard brackets and spring, shackles can thus be retained, allowing modification of existing vehicles.
Each individual leaf spring comprises a spring eye at the first end the leaf spring assembly, and that each spring eye is mounted on a common bushing comprising a central first spacer with a width equal to the predetermined spacing. The individual leaf springs are arranged side-by-side with a predetermined spacing at the intermediate position, separated by second spacer forming part of an axle assembly connecting the axle to the leaf spring assembly.
The invention is also related to a vehicle provided with a suspension comprising at least one leaf spring assembly as described in the above examples.
A vehicle comprising a suspension provided with a leaf spring assembly according to the invention does not require to be secured by a safety system or safety device at each end. This is required for a standard leaf spring in case of spring failure. For a steered front axle, the safety eye provided at the front end to block to front axle dislocation can be eliminated. Similarly, the steel belt or blocker provided at the rear end can be dispensed with. By eliminating this safety system both cost and weight can be decreased.
A spring breakage in a spring assembly according to the invention will still leave one individual leaf spring intact. In this way, an axle dislocation that would normally lead to problems to maintain steering control after a standard leaf spring breakage can be avoided. After a spring breakage the vehicle can still be driven and can make its own way to a service facility.